[PSBT] Clarify that global data can be for inputs and outputs

Clarifies that global data fields redeem scripts, witness scripts,
and hd keypaths can be used for data necessary for both the inputs
and outputs of the transaction.

Author: achow101

bip-0174.mediawiki

@@ -108,7 +108,9 @@ The format of each key-value map is as follows:
 
 The first byte of each key specifies the type of the key-value pair. Some types are
 for global fields and other fields are for each input. The only required type in a
-PSBT is the transaction type, as defined below. The currently defined global types are as follows:
+PSBT is the transaction type, as defined below. All global types that can pertain to both
+inputs and outputs of a transaction can include data for both inputs and outputs of
+the transaction. The currently defined global types are as follows:
 
 {| class="wikitable" style="width: auto; text-align: center; font-size: smaller;
 table-layout: fixed;"
@@ -174,7 +176,8 @@ Value:
 | The public key
 | The master key fingerprint concatenated with the derivation path of the public key. The
 derivation path is represented as 32 bit unsigned integer indexes concatenated
-with each other. This must omit the index of the master key.
+with each other. This must omit the index of the master key. Public keys can be those that
+will be needed to sign any type of key hash input or is spent to by an output.
 | Key: 
 <pre>
 {0x03}|{public key}